Metal halide lamps, a type of discharge lamp, are light sources that seal a metal halide in an arc tube together with mercury and rare gas, and heat the metal halide at high temperature through electric discharge between electrodes of the arc tube to thereby separate the metal halide into metal atoms and halogen atoms and discharge electricity mainly composed of a spectrum that is obtained by exciting the metal atoms. Metal halide lamps have a well-balanced visible radiation spectrum distribution, which gives them excellent color rendering properties, and are fit for many illumination uses.
A certain type of metal halide for metal halide lamps partially remains in a liquid state in the coldest spot of the arc tube while the lamp is lit. The temperature of the coldest spot is therefore changed by changing the posture in which the lamp is lit, and the vapor pressure of the metal halide is accordingly changed as well as the lamp voltage and temperature and light color. Similar changes in characteristics occur for metal halides that are completely evaporated while the lamp is lit since the density distribution in the arc tube changes in accordance with the posture of the lit lamp. This is why a lighting posture in which the optimum characteristics are obtained is specified for metal halide lamps. Low pressure sodium lamps, which have extremely high visible light emission efficiency, possess similar characteristics and therefore take a designated posture when lit (see, for example, Non-patent Document 1).
Various “light flux angle control mirrors” such as condenser mirrors control the divergence of a flux of light emitted from a light source by reflecting. The positional relation between a light flux angle control mirror and a point light source is such that an ideal center line of a flux of light emitted from the point light source and controlled by the mirror runs through the point light source along a sole axis, which is called a “light flux angle control mirror axis”.
Some of those discharge lamps for which a lighting posture is specified are used as illuminators placed in front of a “light flux angle control mirror” to send a flux of light forward beyond the mirror. For many of metal halide lamps and low pressure sodium lamps that are used in this way, the tilt of a “lamp axis”, namely, the line connecting the electrodes of the lamp, with respect to the vertical or horizontal direction is set to a given specified range.
A specific example of a lamp whose tilt with respect to the horizontal direction is set to a given specified range is shown in FIG. 5. A line 54 (lamp axis) connecting electrodes 51 and 53 of a lamp 50 to each other coincides with a lamp length axis 56, and an angle within ±α° from the horizontal line serving as the reference is specified for a lighting posture of the lamp 50.
When a lamp for which a lighting posture is specified and a light flux angle control mirror are arranged limiting the tilt of the “lamp axis” with respect to the vertical direction, the “lamp axis” is set such that the “lamp axis” and the “light flux angle control mirror axis” are orthogonal to each other or substantially coincide with each other. In the former case, the tilt of the center axis of a controlled flux of light is limited with respect to the horizontal direction the same way the tilt of the “lamp axis” is limited with respect to the vertical direction. In the latter case, the same limitation is placed on the tilt of the center axis of a controlled flux of light and the tilt of the “lamp axis”.
When the tilt of the “lamp axis” is limited with respect to the horizontal direction, the “lamp axis” is set such that the “lamp axis” and the “light flux angle control mirror axis” are orthogonal to each other or substantially coincide with each other. In the former case, the center axis of a controlled flux of light is not limited in the up-and-down direction as long as the illuminator is installed in a manner that meets the limitation placed on the “lamp axis” with respect to the horizontal direction. In the latter case, the center axis of a controlled flux of light is limited in the up-and-down direction the same way the “lamp axis” is limited. Non-patent Document 1: “Illumination Handbook”, Ohmsha. Ltd., Jul. 30, 1985 (First Edition, Fifth Printing), p. 179-181